The present invention relates generally to ultrasonic metal welding and, more specifically, to an ultrasonic welding apparatus that reduces sonotrode adhesion during the ultrasonic welding process.
Ultrasonic welding of various materials is known. The process involves vibrating overlapping or adjacent workpieces clamped between a sonotrode and an anvil. Frictional forces occurring between the vibrating workpieces create a bond or weld that occurs at the interface between the workpieces, effectively joining them to one another. Accordingly, various sonotrode and anvil surface configurations, i.e., the surface that contacts the workpieces, are known and used to transfer energy from the sonotrode to the aforementioned interface. Such configurations attempt to reduce the energy loss at the sonotrode/workpiece interface or the anvil/workpiece interface thereby increasing the energy to the workpiece/workpiece interface and increasing the overall efficiency of the ultrasonic welding apparatus.
Further, it is known that when using an ultrasonic welding apparatus to weld light metals, specifically aluminum, the sonotrode or more specifically, the sonotrode adheres to the workpiece being welded. The adhesion can be so severe as to (i) damage the weld when detaching the sonotrode from the joined workpieces, (ii) cause significant and unacceptable distortion of the work piece surface, and (iii) render the sonotrode unusable for subsequent welds. Sticking or adhesion to the workpiece generally results from the sonotrode sliding on the workpiece. When the sonotrode slides, it causes galling or a buildup of material on the sonotrode.
With many of the current sonotrode designs and surface configurations, each time the sonotrode performs a weld, a small amount of aluminum is transferred unto the sonotrode. Continued welding operations cause the aluminum to build up on the sonotrode surface. The built up aluminum on the sonotrode bonds with the material of the workpiece. When this occurs, the sonotrode sticks to, or in short, becomes welded or bonded to the workpiece. Forces of up to 5kN may be required to detach the bonded sonotrode from the workpiece material. Additionally, as aluminum builds up on the sonotrode, it clogs the gripping surface of the sonotrode and reduces the efficiency of the ultrasonic welding apparatus because the energy transferred to the workpiece to perform the weld is reduced.
When the sonotrode becomes clogged, the useful life thereof is reduced. The practical consequence of this is that the sonotrode needs to be cleaned after each weld. Moreover, the surface of the welded material may be severely damaged and will require costly craftsmanship work before it will meet surface finish specifications.
Therefore, there is a need in the art to provide an ultrasonic welding apparatus designed such that it reduces aluminum/sonotrode adhesion during the ultrasonic welding process while improving the productivity, manufacturing speed and reducing equipment downtime by reducing the sticking phenomenon that is common when ultrasonically welding materials.
Accordingly, the present invention is an ultrasonic welding apparatus and method that reduces adhesion between the welding sonotrode and the workpiece during the welding process.
In one embodiment, a fluid is deposited on a contact surface of the sonotrode before the welding process. The fluid may be deposited in several ways all of which are within the scope of the invention. For instance, in one embodiment, the fluid travels through a passageway in the sonotrode to an aperture located on the contact surface of the sonotrode. The fluid is held within the passageway by capillary action. The contact surface may also include at least one groove thereon to aid in distributing the fluid on the contact surface.
In accordance with an additional embodiment, a cooling medium engages the sonotrode and cools the sonotrode below the dew point of the surrounding atmosphere, causing moisture to condense or form on the contact surface of the sonotrode. The cooling medium may be externally blown across the sonotrode or it may travel through various passageways located within the sonotrode.
A further embodiment utilizes the use of cooling fins connected to the sonotrode to cool the sonotrode to a lower temperature, one at which the sonotrode is less likely to stick to the workpiece.
Further, the present invention provides a method for reducing sonotrode adhesion. The method includes several steps operating alone or in combination, including the step of depositing a liquid on the contact surface of the sonotrode. This can be accomplished by using a passageway through the sonotrode to transfer liquid to the contact surface. In addition, the sonotrode and corresponding contact surface can be cooled via a cooling medium, to below the dew point of the surrounding atmosphere thereby causing moisture to condense on the sonotrode.
In addition, cooling the sonotrode before performing the welding process further prevents sonotrode adhesion. Various steps can be taken in to cool the sonotrode including the use of internal cooling passageways in the sonotrode.
Other features and advantages of the present invention will be readily appreciated, as the same becomes better understood after reading the subsequent description taken in conjunction with the accompanying drawings.